In recent years, with rapid spread of electronic devices, housings or the like for, for example, personal computers, mobile phones, and automobile parts have been required to have a conductive function and shield electromagnetic waves to reduce malfunctions of electronic devices and influence on human bodies, etc. Easily moldable thermoplastic resin moldings are increasingly used for IC trays for carrying IC packages. In order to protect ICs from static electricity, thermoplastic resin IC trays need to have electric conductivity and an antistatic function.
Various techniques of sensing distances, speeds, and obstacles using electromagnetic waves are employed. Such cases require shielding of unnecessary electromagnetic waves to perform accurate sensing.
Using conductive resin is one of the methods of making a thermoplastic resin molding conductive to shield electromagnetic waves. However, since conductive resin to be used itself is expensive and impractical, general thermoplastic resin on the market is usually used for a conductive molding. There are roughly two methods of making a general thermoplastic resin molding conductive. One of the methods is to mold thermoplastic resin, and then form a conductive film on the surface of the molding by surface processing such as plating or deposition. The other one is to blend a conductive powder material such as carbon black or metal or a conductive fiber material such as metal fibers or carbon fibers with the thermoplastic resin, and mold the blend.
Out of the two methods, the former of forming the conductive film requires two steps, the molding of the thermoplastic resin, and the surface processing such as plating or deposition. This causes an economical problem in manufacturing. By contrast, the latter of blending the thermoplastic resin with the conductive material requires only one processing, the molding.
A conductive powder or fiber material is blended to thermoplastic resin to make the thermoplastic resin conductive. However, with the same amount, a conductive fiber material has higher conductivity than a conductive powder material. The reason follows. While a conductive function is provided by mutual contact between conductive materials dispersed within thermoplastic resin, conductive fiber materials contact each other more than conductive powder materials. It is thus also understood that a longer conductive fiber material has higher conductivity. As described above, a conductive fiber material may be metal fibers or carbon fibers, for example. Out of them, carbon fibers, particularly long carbon fibers, which have excellent affinity with resin, are particularly preferably used to provide excellent conductivity.
For example, Patent Document 1 discloses a thermoplastic resin molding containing carbon fibers. The amount of the thermoplastic resin in a matrix is 70-99.5 wt %. The total amount of the carbon fibers contained in the molding is 0.5-30 wt %. The content of carbon fibers (a) with lengths greater than 1.5 mm is 0.1-4.7 wt % (with respect to the total amount of the thermoplastic resin and the carbon fibers, the same hereinafter). The content of carbon fibers (b) with lengths of 0.5-1.5 mm is 0.2-10.7 wt %. The content of carbon fibers (c) with lengths smaller than 0.5 mm is 0.2-14.6 wt %. Patent Document 1 teaches that this thermoplastic resin molding provides excellent mechanical characteristics, electric characteristics, and surface appearance, in particular.